The techniques of recombinant protein expression allow for the production of large quantities of desirable proteins which may be used for e.g. their biological activity. Such proteins are often expressed as recombinant fusion proteins in microbial host cells.
The protein of interest is often attached to a fusion partner protein or a smaller amino acid extension in order to increase the expression level, facilitate secretion, increase the solubility, promote protein folding, to protect the protein against unintentional proteolysis or to facilitate purification of the protein of interest. The fusion partner protein needs to be removed from the fusion protein by proteolysis to obtain the protein of interest.
One protease used for such processing is enterokinase (E.C. 3.4.21.9). The biologically natural function of this protease is to convert trypsinogen into trypsin by cleavage at a DDDDK processing site (SEQ ID NO: 2, hereafter D4K) in the zymogen (Biochim. Biophys Acta 20 (1956) 443-434).
Similarly, to enable enterokinase catalysed removal of a fusion partner protein a D4K processing site is inserted between the fusion partner protein and the protein of interest. The specificity and efficiency of the enterokinase catalysed processing of the fusion protein is now dependent of e.g. the relative hydrolysis rates of the D4K site and the potential internal degradation sites in the protein of interest. Enterokinase has activity not only for the D4K site but also for quite a number of other sequences, see e.g. Anal. Biochem. 106 (1980) 199-206.
A number of approaches have been used to remedy the disadvantages of enterokinase having limited substrate specificity.
U.S. Pat. No. 6,906,176 describes a number of peptide sequences which are cleaved more efficiently by enterokinase relative to the D4K site.
PNAS 103 (2006) 7583-7588 describes peptide sequences which are cleaved more rapidly by enterokinase than the D4K site.
Protein Expr. Purif. 41 (2005) 332-340 describes a protein comprising the peptide sequence LKGDR (SEQ ID NO: 3) as being more effective than the D4K site for cleavage by enterokinase.
Protein Expr. Purif. 59 (2008) 314-319 describes the enhancement of the specificity of enterokinase cleavage by conducting the cleavage reaction in the presence of urea.
There is a need for more specific enterokinase cleavage reactions for removing a fusion partner protein without cleaving internal sites in the mature protein and without leaving any amino acid extension on the mature protein. Preferably this enterokinase cleavage reaction is well suited for being carried out during an industrial process for manufacture of the matured protein. There is also a need for a more specific enterokinase cleavage reaction which can be used for many different proteins at mild process conditions such that unintended chemical and physical changes to the mature protein do not occur.